1. Field of the Invention
The present invention relates to wireless communication systems, and in particular to an inter-network handoff method in a code-division multiple access (CDMA) communication system.
2. Background of the Related Art
In the field of wireless communications, several technology-based standards exist for controlling communications between a mobile station (MS) and a base station (BS). Among them, the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series, i.e., IS-95A, IS-95B, and IS-95C (CDMA2000), are digital-based cellular standards entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System.”
An MS using IS-95 series standards constantly searches a Pilot Channel of neighboring BS's for a pilot signal that is sufficiently stronger than a prescribed threshold value. As the MS moves from a region covered by one BS to another, the MS promotes certain pilots from a Neighbor Set to a Candidate Set via a Pilot Strength Measurement Message (PSMM). The BS determines an Active Set of pilot signals according to the PSMM, and notifies the MS of the new Active Set via a Handoff Direction Message (HDM).
Of the various types of handoffs, a soft handoff occurs when the MS starts communication with a new BS in the new Active Set before terminating communications with the old BS. When, on the other hand, the MS starts communication with a new BS in the new Active Set after terminating communications with the old BS, a hard handoff has occurred.
The IS-95A standard was essentially developed for voice communication, and included a maximum single bandwidth of 14.4 Kbps, which was sufficient for voice communication.
Recently, the need for mobile data communication has grown in addition to the need for voice communication, the IS-95 standard has been continuously modified to IS-95B and IS-95C. These improvements provide high speed data services. For example, IS-95b works at 64 Kbps and the IS-95C works at 128 Kbps.
The IS-95B brought improvements over the IS-95A in data transmission speed by occupying 8 traffic channels, so as to support 64 Kbps in circuit service and 115.2 Kbps in packet service. These speeds are 8 times faster than the 8 Kbps and 14.4 Kbps of the IS-95A in respective circuit and packet services.
The IS-95C standard is the most recent version of the IS-95 series, and supports data speeds of up to 144 Kbps, which is the minimum data rate of for IMT-2000.
Generally, a higher version technique is designed to be backward-compatible with lower versions. This is done to protect the current infrastructure investments and experience. Thus, the IS-95C is substantially backward-compatible with the IS-95A/B and other related standards.
However, even though the IS-95C is backward-compatible with the IS-95A/B in voice communications, it is not backward compatible in the data communication. This is due to the large difference between the data rates of the IS-95A/B and IS-95C networks.
Accordingly, in an environment where the IS-95A, IS-95B, and IS-95C networks having different bandwidths coexist, the traffic channels are disconnected when the handoff occurs from the IS-95C network to the IS-95A or IS-95-B network because the IS-95A/B do not support the high date rate of the IS-95C.
Additionally, since the IS-95A/B systems do not support a Fast Forward Power Control, the IS-95A/B systems cannot efficiently perform power control even when the handoff is completed from the IS-95C system to IS-95A/B system.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.